The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
Aspects of the invention relate to a disconnectable mooring system comprising a vessel with an outrigger supporting a riser assembly in a disconnectable manner, which riser assembly is provided with a riser top body which by means of disconnectable latch is attached to the outrigger.
In offshore oil production, floating production units such as for example vessels are employed to receive effluents from subsea wells. To achieve this, riser assemblies comprising flexible risers with a riser top body are usually employed to connect such wells with the floating production units. In most cases, such floating production units are permanently anchored in the field until its depletion. In some cases however, weather conditions such as severe storms may make it necessary that the floating production facility vacates the field temporarily until the weather conditions improve again. In such case the floating production unit stops production, closes the valves on the wells, and disconnects the riser assembly from the unit. The riser assembly is left in the field to survive the storm on its own. In such case it is important that a proper riser assembly configuration is established such that the riser assembly does not for example get entangled in itself and suffers any damage.
It is also important that the actual riser assembly disconnect and abandonment system is properly designed to allow a safe release of the riser assembly from the floating production unit. Usually this is done by a winch which lowers the more or less buoyant end termination (riser top body) of the riser assembly into the water. After that, the winch wire is released from the winch.
An example of a disconnectable mooring system of the above type is described in U.S. Pat. No. 5,041,038.
With a shift in oil production towards deeper waters and towards the use of more and heavier flexible risers, as well as an increase in the use of dynamically positioned, weathervaning floating production units, the combined loads exerted by the riser assemblies on the disconnect facility become very large. Weathervaning units generally do not have sufficient space to allow the use of individual release connectors and deep waters and large riser assemblies also imply substantial weight of buoyancy means to be carried by the connector and ultimately to be lowered over board.
All of the above lead to the fact that the winch used to haul in and connect the riser assembly to the floating production unit, is generally not capable of lowering the riser assembly but very slowly, with a line speed similar to the pull-in speed. Since usually during disconnecting the wave heights are more severe than during the pull-in, slow release speeds mean that a significant potential for interference exists between the disconnecting elements due to wave action. This in not desirable as it may lead to damage to both the floating production unit as well as the riser top body.
On the other side, while a pure free-fall would be ideal to achieve a quick separation, this is no longer possible since the larger weights of the riser top body would, if released in a free-fall mode, cause the lower-lying parts of the individual risers to experience compression and even buckling.
This is due to the fact that while the disconnect mechanism, including riser top buoyancy assembly, are preferably located above water, the rest of the riser is largely located in the water and hence cannot, due to drag caused by the surrounding water, move quickly enough ahead on the trajectory to be followed by the riser top body.